Conventionally, as display elements included in a display device, there have been an electro-optical element of which luminance is controlled by an applied voltage, and an electro-optical element of which luminance is controlled by a flowing current. As a representative example of the electro-optical element of which luminance is controlled by the applied voltage, there is a liquid-crystal display element. Meanwhile, as a representative example of the electro-optical element of which luminance is controlled by the flowing current, there is an organic EL (electro-luminescence) element. The organic EL element is also referred to as an OLED (Organic Light-Emitting Diode). The organic EL display device using the organic EL element which is a self light-emitting type electro-optical element can easily achieve thinning, low power consumption, high luminance, and the like, as compared with a liquid crystal display device which requires backlight, a color filter, and the like. Therefore, in recent years, development of the organic EL display device has been positively progressed.
As a driving system of the organic EL display device, there have been known a passive matrix system (also referred to as a simple matrix system) and an active matrix system. The organic EL display device employing the passive matrix system has a simple structure, but it is difficult to achieve size increase and definition enhancement. Meanwhile, the organic EL display device employing the active matrix system (hereinafter, referred to as an “active matrix-type organic EL display device”) can easily realize size increase and definition enhancement as compared with the organic EL display device employing the passive matrix system.
The active matrix-type organic EL display device includes a plurality of pixel circuits arranged in a matrix. Each of the pixel circuits of the active matrix-type organic EL display device typically includes an input transistor for selecting a pixel, and a driving transistor for controlling a supply of a current to the organic EL element. Note that in the following, a current flowing from the driving transistor to the organic EL element is also referred to as a “driving current”.
Ina general active matrix-type organic EL display device, one pixel includes three sub-pixels (an R sub-pixel which displays a red color, a G sub-pixel which displays a green color, and a B sub-pixel which displays a blue color). FIG. 26 is a circuit diagram illustrating a configuration of a conventional general pixel circuit 91 configuring one sub-pixel. The pixel circuit 91 is provided corresponding to each of intersections of a plurality of data lines DL and a plurality of scanning signal lines SL which are arranged in a display unit. As illustrated in FIG. 26, the pixel circuit 91 includes two transistors T1 and T2, one capacitor Cst, and one organic EL element OLED. The transistor T1 is a driving transistor, and the transistor T2 is an input transistor. It should be noted that, in the example illustrated in FIG. 26, the transistors T1 and T2 are n-channel thin film transistors (TFT).
The transistor T1 is provided in series with the organic EL element OLED. Concerning the transistor T1, a drain terminal is connected to a power supply line for supplying a high-level power supply voltage ELVDD (hereinafter, referred to as a “high-level power supply line”, and designated with the same symbol ELVDD as that given to the high-level power supply voltage), and a source terminal is connected to an anode terminal of the organic EL element OLED. The transistor T2 is provided between a data line DL and a gate terminal of the transistor T1. Concerning the transistor T2, a gate terminal is connected to a scanning signal line SL, and a source terminal is connected to the data line DL. Concerning the capacitor Cst, one end is connected to the gate terminal of the transistor T1, and the other end is connected to the source terminal of the transistor T1. A cathode terminal of the organic EL element OLED is connected to a power supply line for supplying a low-level power supply voltage ELVSS (hereinafter, referred to as a “low-level power supply line”, and designated with the same symbol ELVSS as that given to the low-level power supply voltage). Hereinafter, a connection point of the gate terminal of the transistor T1, one end of the capacitor Cst, and the drain terminal of the transistor T2 will be referred to as a “gate node VG” for convenience (this is similarly applied to FIG. 1 and FIG. 17). It should be noted that, in general, one of the drain and the source having a higher potential is called a drain. However, in the description of the present specification, one is defined as a drain, and the other is defined as a source. Therefore, a source potential becomes higher than a drain potential in some cases.
FIG. 27 is a timing chart for describing the operation of the pixel circuit 91 illustrated in FIG. 26. Before time t1, the scanning signal line SL is in an unselected state. Therefore, before the time t1, the transistor T2 is in an OFF state, and the potential of the gate node VG maintains an initial level (a level according to writing in one preceding frame, for example). At the time t1, the scanning signal line SL becomes in a selected state, and the transistor T2 is turned on. Accordingly, via the data line DL and the transistor T2, a data voltage Vdata corresponding to the luminance of a pixel (a sub-pixel) formed by the pixel circuit 91 is supplied to the gate node VG. Thereafter, during a period till time t2, the potential of the gate node VG changes according to the data voltage Vdata. At this time, the capacitor Cst is charged to a gate-source voltage Vgs which is a difference between the potential of the gate node VG and the source potential of the transistor T1. At the time t2, the scanning signal line SL becomes in an unselected state. Accordingly, the transistor T2 is turned off, and the gate-source voltage Vgs held by the capacitor Cst is established. The transistor T1 supplies a driving current to the organic EL element OLED according to the gate-source voltage Vgs held by the capacitor Cst. As a result, the organic EL element OLED emits light with the luminance corresponding to the driving current.
Incidentally, the pixel circuit 91 illustrated in FIG. 26 is a circuit corresponding to one sub-pixel. Therefore, a pixel circuit 910 corresponding to one pixel including three sub-pixels has a configuration as illustrated in FIG. 28. As illustrated in FIG. 28, the pixel circuit 910 configuring one pixel includes a pixel circuit 91(R) for an R sub-pixel, a pixel circuit 91(G) for a G sub-pixel, and a pixel circuit 91(B) for a B sub-pixel. Note that in the following, the configuration illustrated in FIG. 28 will be referred to as a “first conventional example”. As can be understood from FIG. 28, according to the first conventional example, six transistors and three capacitors are necessary per one pixel.
In the case of a configuration requiring many circuit elements in the pixel circuit, it is difficult to achieve definition enhancement. Therefore, Japanese Patent Application Laid-Open No. 2005-148749 discloses a pixel circuit 920 having a configuration in which numbers of transistors and capacitors necessary per one pixel are smaller than numbers necessary in the first conventional example, as illustrated in FIG. 29. Note that in the following, the configuration illustrated in FIG. 29 will be referred to as a “second conventional example”. The pixel circuit 920 in the second conventional example includes a driving means 921, a sequential control means 922, and three organic EL elements OLED(R), OLED(G), and OLED(B). The driving means 921 includes a driving transistor T11, an input transistor T12, and a capacitor Cst1. The sequential control means 922 includes a transistor T13(R) for controlling light emission of the red-color organic EL element OLED(R), a transistor T13(G) for controlling light emission of the green-color organic EL element OLED(G), and a transistor T13(B) for controlling light emission of the blue-color organic EL element OLED(B).
In the above configuration, one frame period is divided into an R sub-frame for emitting red color light, a G sub-frame for emitting green color light, and a B sub-frame for emitting blue color light. Then, in the sequential control means 922, only the transistor T13(R) is set to an ON state in the R sub-frame, only the transistor T13(G) is set to an ON state in the G sub-frame, and only the transistor T13(B) is set to an ON state in the B sub-frame. Thus, over the one frame period, the organic EL element OLED(R), the organic EL element OLED(G), and the organic EL element OLED(B) are caused to sequentially emit light so that a desired color image is displayed. It should be noted that ON/OFF of the transistors T13(R), T13(G), and T13(B) is controlled by light-emission control signals given respectively to light-emission control lines 923(R), 923(G), and 923(B). According to the second conventional example, five transistors and one capacitor are necessary per one pixel.
Japanese Patent Application Laid-Open No. 2005-148750 discloses a pixel circuit 930 having a configuration in which numbers of data lines and capacitors are smaller than numbers necessary in the first conventional example, as illustrated in FIG. 30. Note that in the following, the configuration illustrated in FIG. 30 will be referred to as a “third conventional example”. As can be understood from FIG. 30, according to the third conventional example, although the number of transistors becomes larger than that necessary in the first conventional example, the numbers of data lines and capacitors become smaller than those necessary in the first conventional example.